When compared to conventional “Vee” and straight-inline internal combustion engines with a single piston in each cylinder, opposed-piston engines possess architectural advantages in thermodynamics and combustion that can deliver improvements in measures of engine performance. However, uniflow-scavenged, opposed-piston engines characteristically have thermal requirements that are different from engines that have one piston per cylinder, and thus thermal management techniques differ. The difference in thermal requirements occurs in uniflow-scavenged opposed-piston engines due to the nature of charge air flow into and exhaust flow from the cylinders in these engines.
During scavenging in a uniflow-scavenged, opposed-piston engine, the predominant fluid flow is unidirectional, that is to say, charge air flows through the intake port of a cylinder and exhaust flows out of the cylinder's exhaust port. Because the air entering the cylinder is cooler than the exhaust, the exhaust portion of the cylinder and the exhaust piston that moves across the exhaust port are exposed to greater heat and higher temperatures than the intake portion of the cylinder and the intake piston that moves across the intake port. Thus, the unidirectional flow of air and exhaust leads to exposure of the opposite ends of a cylinder to different temperature profiles. Additionally, in a two-stroke cycle of uniflow-scavenged, opposed-piston engines, there is less time for piston cooling between firing or combustion events, so the difference in thermal environments that the exhaust and intake pistons are exposed to is even more pronounced as compared to engines that have one piston per cylinder.